Poor solderability of printed wiring boards (PWBs) or printed circuit boards (PCBs) and their associated component leads is a difficulty encountered throughout the U.S. Department of Defense as well as throughout private industry. A primary cause for solderability degradation of PWBs is oxidation of the base metal and/or the oxidation of coatings applied to or present over said base metals. Solderability can be retarded by the presence of oxide films on the base metals or protective coatings. This oxide film may be formed by available and present natural resources, such as small amounts of oxygen in the atmosphere.
Currently, fluxes are used during the soldering process in an attempt to resolve the problem. Fluxes are designed to remove surface oxides and then to protect the surface from reoxidation during subsequent soldering processes. However, it has been found that even after fluxes are employed, highly oxidized base metals or coatings experienced degradation in solderability. Therefore, even when fluxes are employed, some method to pre-gauge a PWB, PCB, or a component lead for solderability is essential.
Currently, tin-lead and some other metallic materials are used as protective coatings to prolong solderability of base materials. A problem, however, arises with the use of metallic protective coatings in that these coatings are subject to oxidation and the growth of intermetallics, both of which can detrimentally effect the solderability properties of a substrate.
Since poor solderability of PWBs and PCBs remains a problem, the invention herein will at a minimum simplify the method of detecting this problem.
Currently, solderability of a PWB is measured by testing coupons attached to the PWB while components are tested by testing their actual leads. This is presently accomplished by applying solder to the coupon or component lead and then examining it for a variety of parameters such as area of spread, dewetting, wetting force and wetting time. For PWBs, this is an indirect test and does not give a true solderability measure of the board itself. For components, it requires the actual application of solder to the leads prior to the actual soldering step. Moreover, these tests tend to be subjective in nature and therefore not very reliable.
Another procedure presently under development for testing and measuring the solderability of a circuit board is a technique called electrochemical reduction. Although this experimental technique does not involve the application of solder onto the component part to be evaluated, it requires the application of a reducing agent to remove surface oxides and physically affects the component part.
The principle of monitoring an oxide coating thickness is the topic of G W. Poling, "Infrared Reflection Studies of the Oxidation of Copper and Iron," Journal of electrochemical Society, Vol. 116, No. 7 (July, 1969) and C. W. Shanley, "Differential Reflectometry--A New Optical Technique to Study Corrosion Phenomena," Corrosion Science, Vol. 20 (1980). These articles show a direct relationship between the intensity of the reflectance spectra of copper and the thickness of cuprous and cupric oxide on the copper surface. This teaching additionally supports the axiom that the reflectance of, for example, a printed circuit board can be used to monitor the oxide thickness present. It nowhere, however, teaches the use of reflectance to measure solderability qualities of a substrate.
As set forth above, the existing solderability testing techniques have their limitations. The present invention, the method of gauging solderability, does not have any adverse limitations on its operability. The present methodology for gauging solderability is a procedure that may be used on various base metals and coatings in conjunction with various solders. It does not involve the application of any solder and does not physically alter the component to be evaluated. Moreover, the present invention tests the component directly and does not require the use of test coupons. Use of the present invention facilitates the thorough, expeditious and efficient inspection of the solderability property of printed circuit boards.
To date, a method of directly determining the solderability of a circuit board which has no adverse or physical affect to the component board itself has not been proposed.